1. Field of the Invention
The present invention relates to a color proofing apparatus which utilizes an electronic signal input, and more particularly, to a method and apparatus for automatically producing full-color proof images using lasers to provide thermal energy to a series of color dye-donors to selectively transfer each dye in registration to a receiver to form a proof image.
2. Description of the Prior Art
Color-proofing is the procedure used by the printing industry for creating representative images that replicate the appearance of printed images without the cost and time required to actually set up a high-speed, high-volume printing press to print an example of the images intended. In the past, these representative images, or proofs, have been generated from the same color-separations used to produce the individual color printing plates used in printing presses so that variations in the resulting images can be minimized. Various color-proofing systems have been devised to create the proofs and have included the use of smaller, slower presses as well as means other than presses, such as photographic, electrophotographic, and non-photographic processes.
The proofs generated are judged for composition, screening, resolution, color, editing, and other visual content. The closer the proof replicates the final image produced on the printing press, as well as the consistency from image to image, from press to press, and from shop to shop, the better the acceptance of the proofing system by the printing industry. Other considerations used in judging proofing systems include reproducibility, cost of the system as well as cost of the individual proofs, speed, and freedom from environmental problems. Further, since nearly all printing presses utilize the half-tone process for forming pictorial images, wherein the original image is screened, i.e. photographed through a screen to produce one or more printing plates containing an image formed of a plurality of fine dots that simulate the varying density of the original image, proofing processes that employ the half-tone process to form the proof image have been better accepted by the printing industry than have continuous tone systems.
With the advent of electronic systems for the generation of printing plates from electronic data stored in appropriate data storage devices in the form of electronically separated single color separations, the use of photographic color separations for generating proof images has become somewhat archaic, and a variety of processes have been developed and implemented to electronically form, store, and manipulate images both for generating the actual printing plates as well as for generating the proof images. While some of these electronic systems can handle and produce analog images, the most widely used systems employ digital processes because of the ease of manipulation of such digital images. In each of these electronic processes it is possible to display the resulting image on a CRT display, but it is generally necessary to produce a "hard copy" (i.e. an image actually formed on a sheet of paper or other material) before it can be fully assessed for approval of the final printing operation. Thus, each of these electronic systems requires the use of some form of output device or printer which can produce a hard copy of the image for actual evaluation. It is to the field of proofing output devices that the present invention is directed.
While purely photographic processes can provide accurate reproductions of images, they do not always replicate the reproduction resulting from printing presses. Further, most photographic processes do not produce half-tone images that can be directly compared to the printed images they are intended to emulate. Moreover, they are almost universally incapable of reproducing the images on the wide variety of paper or other material that can be run through a printing press. It is known that the appearance of the final printed image is affected by the characteristics of the paper or other material upon which it is printed. Thus, the ability to form the proof image on the material actually to be used in the press can be a determining factor in the market success of the proofing system.
Other continuous tone proofing systems, such as thermal processes and ink-jet systems have been developed, but they do not replicate the half-tone images so desired by the printing industry.
Electrophotographic proofing systems with half-tone capability have been introduced which employ either wet or dry processes. The electrophotographic systems that use dry processes suffer from the lack of high resolution necessary for better quality proofing, particularly when the images are of almost continuous tone quality. This results from the fact that dry electrophotographic processes do not employ toner particles which have a sufficiently small size to provide the requisite high image resolution. While wet electrophotographic processes do employ toners with the requisite small particle size, they have other disadvantages such as the use of solvents that are environmentally undesirable.
In commonly assigned U.S. patent application Ser. Nos. 451,655, U.S. Pat. No. 5,164,742 issued Nov. 17, 1992 and 451,656, U.S. Pat. No. 5,168,288 issued Dec. 1, 1992, both filed Dec. 18, 1989, a thermal printer is disclosed which may be adapted for use as a direct digital color proofer with half-tone capabilities. This printer is arranged to form an image on a thermal print medium, or writing element, in which a donor element transfers a dye to a receiver element upon receipt of a sufficient amount of thermal energy. This printer includes a plurality of diode lasers which can be individually modulated to supply energy to selected areas of the medium in accordance with an information signal. The print-head of the printer includes one end of a fiber optic array having a plurality of optical fibers coupled to the diode lasers. The thermal print medium is supported on a rotatable drum, and the print-head with the fiber optic array is movable relative to the drum. The dye is transferred the receiver element as the radiation, transferred from the diode lasers to the donor element by the optical fibers, is converted to thermal energy in the donor element.
A direct digital color proofer utilizing a thermal printer such as that just described must be capable of consistently and accurately writing minipixels at a rate of 1800 dots per inch (dpi) and higher to generate half-tone proofs having a resolution of 150 lines per inch and above, as is necessary to adequately proof high quality graphic arts images such as those found in high quality magazines and advertisements. Moreover, it is necessary to hold each dot or minipixel to a density tolerance of better than 0.1 density unit from that prescribed in order to avoid visible differences between the original and the proof. This density control must be repeatable from image-to-image and from machine-to-machine. Moreover, this density control must also be maintained in each of the colors being employed in multiple passes through the proofer to generate a full color image.
Aspects of the apparatus which affect the density of the dots that make up the image include such things as variations and randomness of the intensity and frequency of the laser output, and variations in the output of the fiber optics which can vary from fiber to fiber and even within a single fiber as it is moved during the writing process. Variations in the finish of the drum surface as well as drum runout and drum bearing runout and variations in the parallelism of the translation of the print-head with respect to the axis of the drum also affect the density of the image dots. Any uneven movement of the imaging drum or of the writehead translation during the writing process, or anything which imparts jitter to any part of the imaging apparatus can adversely impact the quality of the finished image and its value as a representative proof. The difference in the distance between the ends of individual fibers and the drum surface also affects image density because of the fact that the end of the fiber bundle is flat while the surface of the drum is curved. Temperature variations in the print-head due to the ambient temperature of the machine as well as the fact that the writing process itself heats the print-head also influence the image density.
Variations in the print medium elements, such as variations in the thickness of the donor and receiver elements as well as the various layers that are a part thereof, can also affect the image density as it is being written.
Thus, it has been found necessary to provide a writing apparatus which meets all of the foregoing requirements and yet which is substantially automated to improve the control, quality and productivity of the proofing process while minimizing the attendance and labor necessary. Moreover, the writing apparatus must be capable of not only generating this high quality image consistantly, but it must be capable of creating a multi-color image which is in registration regardless of how the various individual images are supplied to the element comprising the final image. That means, in a thermal proofing process such as that described above, that various donor material sheets must be sequentially superposed with a single receiver sheet and then removed without disturbing the receiver sheet on the writing drum or platen, since maintaining the receiver sheet in one position during the entire writing process is what assures the necessary registration of the multiple superposed images that create the final proof.
Thus it will be seen that a method and apparatus for constantly, quickly and accurately generating an image utilizing such a thermal imaging process to create high quality, accurate, and consistant proof images would be technologically desirable and economically advantageous.